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- Volcano-tectonic evolution of a linear volcanic ridge (Pico-Faial Ridge, Azores Triple Junction) assessed by paleomagnetic studiesPublication . Silva, Pedro; Henry, Bernard; Marques, Fernando Ornelas; Hildenbrand, Anthony; Lopes, Ana; Madureira, Pedro Miguel; Madeira, JFA; Nunes, João C.; Roxerová, ZuzanaThe morphology of volcanic oceanic islands results from the interplay between constructive and destructive processes, and tectonics. In this study, the analysis of the paleomagnetic directions obtained on well-dated volcanic rocks is used as a tool to assess tilting related to tectonics and large-scale volcano instability along the Pico-Faial linear volcanic ridge (Azores Triple Junction, Central-North Atlantic). For this purpose, 530 specimens from 46 lava flows and one dyke from Pico and Faial islands were submitted to thermal and alternating magnetic fields demagnetizations. Detailed rock magnetic analyses, including thermomagnetic analyses and classical high magnetic field experiments revealed titanomagnetites with different Ti-content as the primary magnetic carrier, capable of recording stable remanent magnetizations. In both islands, the paleomagnetic analysis yields a Characteristic Remanent Magnetization, which presents island mean direction with normal and reversed polarities in agreement with the islands location and the age of the studied lava flows, indicating a primary thermo-remanent magnetization. Field observations and paleomagnetic data show that lava flows were emplaced on pre-existing slopes and were later affected by significant tilting. In Faial Island, magmatic inflation and normal faults making up an island-scale graben, can be responsible for the tilting. In Pico Island, inflation related to magma intrusion during flow emplacement can be at the origin of the inferred tilting, whereas gradual downward movement of the SE flank by slumping processes appears mostly translational.
- The 1998-2001 submarine lava balloon eruption at the Serreta ridge (Azores archipelago): constraints from volcanic facies architecture, isotope geochemistry and magnetic dataPublication . Madureira, Pedro Miguel; Rosa, Carlos; Marques, Ana Filipa; Silva, Pedro; Moreira, Manuel; Hamelin, Cedric; Relvas, Jorge; Lourenço, Nuno; Conceição, Patrícia; de Abreu, Manuel Pinto; Barriga, Fernando J. A. S.The most recent submarine eruption observed offshore the Azores archipelago occurred between 1998 and 2001 along the submarine Serreta ridge (SSR), similar to 4-5 nautical miles WNW of Terceira Island. This submarine eruption delivered abundant basaltic lava balloons floating at the sea surface and significantly changed the bathymetry around the eruption area. Our work combines bathymetry, volcanic facies cartography, petrography, rock magnetism and geochemistry in order to (1) track the possible vent source at seabed, (2) better constrain the Azores magma source(s) sampled through the Serreta submarine volcanic event, and (3) interpret the data within the small-scale mantle source heterogeneity framework,that has been demonstrated for the Azores archipelago. Lava balloons sampled at sea surface display a radiogenic signature, which is also correlated with relatively primitive (low) He-4/He-3 isotopic ratios. Conversely, SSR lavas are characterized by significantly lower radiogenic Sr-87/Sr-86,Pb-206/Pb-204 and Pb-208/Pb-204 ratios than the lava balloons and the onshore lavas from the Terceira Island. SSR lavas are primitive, but incompatible trace-enriched. Apparent decoupling between the enriched incompatible trace element abundances and depleted radiogenic isotope ratios is best explained by binary mixing of a depleted MORB source and a HIMU-type component into magma batches that evolved by similar shallower processes in their travel to the surface. The collected data suggest that the freshest samples collected in the SSR may correspond to volcanic products of an unnoticed and more recent eruption than the 1998-2001 episode. (C) 2016 Elsevier B.V. All rights reserved.